The invention relates to a display device comprising at least three light panels for emitting at least three light beams having different colors, combination means which are rigidly connected to a frame for combining the three light beams to a combined light beam for projection on a screen, positioning means for spatially positioning the three light panels, and fixation means for fixing a position, adjusted with the aid of the positioning means, of the individual light panels with respect to the combination means.
Within the scope of this invention, the term light panel is to be understood to mean at least transmissive liquid crystal displays or LCDs, reflective liquid crystal displays, or R-LCDs, and digital mirror devices, or DMDs. Such optical components are generally known to those skilled in the art and do not need any further explanation.
In display devices according to the invention, in which a plurality of light beams is combined to a combined light beam for projection on a screen, it is of great importance that the relevant light panels are positioned correctly with respect to the combination means and with respect to each other in order to obtain a sharply focused image on the screen, while the light beams converge in the correct manner. Since the light panels are considerably smaller as compared with the ultimate image, which may have a main dimension of 35 mm, it is necessary to position the light panels within a tolerance of approximately xc2x15 micrometers with respect to the combination means, such as a prism. Since it is economically not feasible to obtain such accuracies as measuring tolerances for the components of the display devices, it is known to correctly position the light panels with respect to the frame by means of an external manipulator and by subsequently fixing this position.
United States patent U.S. Pat. No. 5,653,522 describes a display device as mentioned in the opening paragraph, in which a transmissive light panel is secured to mounting brackets by means of three fixation elements, a first fixation element in the form of a rod being rigid in all directions along the surface area of the light panel, a second fixation element in the form of a clamp being rigid in a direction of the light panel, and a third fixation element, also in the form of a clamp being rigid in a direction perpendicular to the rigid direction of the second fixation element. Such a way of fixation envisages absorption of thermal expansions during heating without stress in the construction. During manufacture of the display device, the fixation elements are maneuvered into their correct position after which they are rigidly connected to the relevant light panel by means of an adhesive which cures under the influence of ultraviolet light. The drawback of such a fixation method is that manipulation of the light panel is only possible to a limited extent so that the tolerance requirements which are imposed on the production process of the individual components as well as on the assembly process are relatively stringent. This manipulation is notably possible in a limited manner in the direction which is necessary for focusing the image.
It is an object of the invention to provide a display device which offers a solution, either or not in a preferred embodiment. In addition, the display device should be defined completely statically so that the risk of mechanical stress in the light panel is reduced considerably. Such stress might give rise to a disturbance of the image. To this end, the invention is characterized in that the positioning means comprise three plate-shaped elements for each light panel, which elements are oriented at right angles to the associated light panel and are each rigidly connected to the frame along a connecting edge, and are each pivotally connected to the associated light panel at a distance from the connecting edge. Within the scope of this invention, pivotal is to be understood to mean the pivotability to an extent which is necessary for correctly positioning the light panels.
Plate-shaped elements which are rigidly incorporated in a frame along an edge, such as a leaf spring, have the property that they have a great rigidity as regards the three degrees of freedom (twice translation, once rotation) extending in the plane of the plate-shaped element. The three other degrees of freedom (once translation, twice rotation) extending from the plane of the plate-shaped element have, however, a relatively small rigidity. The invention advantageously utilizes this property by transferring these properties of rigidity to the light panel by means of the pivotal connection. An exception is the relatively rigid rotation in the plane of the plate-shaped element. Due to the pivotal character of the connection between the plate-shaped element and the associated light panel, the light panel does not behave rigidly as regards rotation about an axis perpendicular to the plate-shaped element. Limited movements of the light panel, as are required for focusing and converging the light panels, are possible in principle, in so far as these take place perpendicularly to the plane of the plate-shaped elements. In principle, also the rotation of the light panel may take place in the plane of the plate-shaped element. Per plate-shaped element, two degrees of freedom are thus taken from the associated light panel. By using three plate-shaped elements per light panel, all of the six degrees of freedom of the light panel can thus be fixed. After the light panel has been brought to the correct spatial position with respect to the combination means and the other light panels by way of manipulation, this correct position is fixed by the action of the fixation means.
By orienting the plate-shaped elements at an angle to the associated light panel, an optimum rigidity of the connection between light panel and plate-shaped elements is obtained.
The same also applies if two of the plate-shaped elements are oriented perpendicularly to each other.
If two of the plate-shaped elements are oriented parallel to each other, They can be caused to coincide with two opposite edges of the light panel.
In accordance with a special preferred embodiment, all plate-shaped elements substantially extend within the perpendicular projection of the associated light panel. A compact construction is obtained in this way.
If at least one, and preferably all plate-shaped elements, are connected to the associated light panel on the side facing the connecting edge, optimum use is made of the material of the plate-shaped elements so that the ratio between the occupation of the space and the rigidity obtained remains as small as possible.
For the pivotal connection between the plate-shaped element and the associated light panel, each plate-shaped element is preferably provided with a lateral projection extending rotatably into a recess of the associated light panel. Such a projection may be provided, for example, immediately during punching of the plate-shaped elements.
The projection is connected to the recess preferably by means of a flexible adhesive joint. The flexible adhesive joint allows pivoting to an extent which is sufficient for focusing and converging the relevant light panel.
In accordance with a special preferred embodiment, the fixation means comprise clamping means for each plate-shaped element for detachably and rigidly connecting the connecting edge of the plate-shaped element to the frame. The detachable character of such a connection provides the possibility of separately replacing defective light panels.
A particularly simple embodiment of such clamping means is obtained if they are substantially U-shaped. The clamping surfaces are then formed by the inner side of the legs of the U, while the underlying back of the U provides the possibility of bending both legs towards each other.
The clamping means preferably have a length which is at least as large as the length of the connecting edge so that an optimum rigid connection is obtained along this connecting edge which does not allow any rotation of the clamped plate-shaped element about an axis perpendicular to the plane of the plate-shaped element.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.